Controlled power dissipation in a link path in a lighting system

ABSTRACT

In at least one embodiment, a lighting system includes a link path power dissipation circuit to actively and selectively control power dissipation of excess energy in a switching power converter of the lighting system. The link path power dissipation circuit dissipates power through a link path of the switching power converter by controlling a link current of the switching power converter. In at least one embodiment, the controller controls the link path power dissipation circuit to limit the link current with a current source and dissipate power in the current source. In at least one embodiment, the link path power dissipation circuit includes a switch to limit the link current and dissipate power in the link path.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(e) and 37C.F.R. §1.78 of U.S. Provisional Application No. 61/410,168, filed onNov. 4, 2010, which is incorporated by reference in its entirety. Thisapplication also claims the benefit under 35 U.S.C. §120 and 37 C.F.R.§1.78 of U.S. patent application Ser. No. 13/289,845, filed Nov. 4,2011, which is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to the field of electronics,and more specifically to a method and system for controlling energydissipation in a link path of a lighting system.

2. Description of the Related Art

Switching power converters convert power received from a power source,such as a voltage supply, into power suitable for a load. The powerreceived from the voltage supply is referred to as “POWER IN”, and thepower provided to the load is referred to as “POWER OUT”. All switchingpower converters have some inherent power losses due to, for example,non-ideal component characteristics. Such inherent power losses tend tobe minimized so as to increase the efficiency of the switching powerconverters. Inherent power losses are represented herein by “P_(INH)”.In some contexts the amount of power supplied to the switching powerconverter can exceed the amount of power provided by the switching powerconverter to a load, i.e. POWER IN>POWER OUT+P_(INH). When the POWER INis greater than the POWER OUT+P_(INH), the switching power converterpassively dissipates the excess energy using passive resistors.

A dimmable lighting system that includes a low power lamp, such as oneor more light emitting diodes (LEDs), represents one context when thePOWER IN to the switching power converter can be greater than the POWEROUT P_(INH) of the switching power converter. In this exemplary context,the switching power converter receives current through a triode foralternating current (“triac”) based dimmer. Once a triac-based dimmerbegins conducting during a cycle of an alternating current (“AC”) supplyvoltage to prevent the triac from disadvantageously, prematurelydisconnecting during mid-cycle of the supply voltage, the switchingpower converter draws a minimum current referred to as a “hold current”.As long as an input current to the switching power converter is greaterthan or equal to the hold current, the triac-based dimmer should notprematurely disconnect. For a leading edge dimmer, a prematuredisconnect occurs when the dimmer begins conducting and stops conductingprior to reaching a zero crossing of the supply voltage. Prematuredisconnects can cause problems with the lighting system, such as flickerand instability.

Thus, to prevent premature disconnection of the triac-based dimmer, theminimum POWER IN to the switching power converter equals the holdcurrent (“i_(HOLD)”) times an input voltage “V_(IN)” to the switchingpower converter. Conventional triac-based dimmers were designed toprovide power to incandescent light bulbs. For desired dimming levels,an incandescent light bulb generally draws a current at least equal tothe hold current for all usable dimming levels. However, other lamps,such as LEDs are more efficient than incandescent light bulbs in termsof power versus light output and, thus, provide equivalent light outputwhile using less power than an incandescent light bulb. Thus, lightingsystems with LEDs typically utilize less power and less current thanincandescent bulbs. To balance the power when the lighting system drawsmore POWER IN than the lighting system inherently dissipates andprovides as POWER OUT to the lamp, the lighting system utilizes one ormore passive resistors to internally dissipate excess power.

FIG. 1 depicts a lighting system 100 that includes a leading edge,phase-cut dimmer 102. FIG. 2 depicts ideal, exemplary voltage graphs 200associated with the lighting system 100. Referring to FIGS. 1 and 2, thelighting system 100 receives an AC supply voltage V_(IN) from voltagesupply 104. The supply voltage V_(IN), indicated by voltage waveform202, is, for example, a nominally 60 Hz/110 V line voltage in the UnitedStates of America or a nominally 50 Hz/220 V line voltage in Europe. Aleading edge dimmer 102 phase cuts leading edges, such as leading edges204 and 206, of each half cycle of supply voltage V_(IN). Since eachhalf cycle of supply voltage V_(IN) is 180 degrees of the input supplyvoltage V_(IN), the leading edge dimmer 102 phase cuts the supplyvoltage V_(IN) at an angle greater than 0 degrees and less than 180degrees. Generally, the voltage phase cutting range of a leading edgedimmer 102 is 10 degrees to 170 degrees. “Phase cutting” the supplyvoltage refers to modulating a leading edge phase angle of each cycle ofan alternating current (“AC”) supply voltage. “Phase cutting” of thesupply voltage is also commonly referred to as “chopping”. Phase cuttingthe supply voltage reduces the average power supplied to a load, such asa lighting system, and thereby controls the energy provided to the load.

The input signal voltage V_(φ) _(_) _(IN) to the lighting system 100represents a dimming level that causes the lighting system 100 to adjustpower delivered to a lamp 122, and, thus, depending on the dimminglevel, increase or decrease the brightness of the lamp 122. Manydifferent types of dimmers exist. In general, dimmers use a digital oranalog coded dimming signal that indicates a desired dimming level. Forexample, the triac-based dimmer 102 phase cuts a leading edge of the ACinput supply voltage V_(IN). The leading edge dimmer 102 can be any typeof leading edge dimmer such as a triac-based leading edge dimmeravailable from Lutron Electronics, Inc. of Coopersberg, Pa. (“Lutron”).A triac-based leading edge dimmer is described in the Background sectionof U.S. patent application Ser. No. 12/858,164, entitled Dimmer OutputEmulation, filed on Aug. 17, 2010, and inventor John L. Melanson.

The phase cut dimmer 102 supplies the input voltage V_(φ) _(_) _(IN) asmodified by the phase cut dimmer 102 to a full bridge diode rectifier106. The full bridge rectifier 106 supplies an AC rectified voltageV_(φR) _(_) _(IN) to the switching power converter 108. Capacitor 110filters high frequency components from rectified voltage V_(φ) _(_)_(IN). To control the operation of switching power converter 108,controller 110 generates a control signal CS₀ to control conductivity offield effect transistor (FET) switch 112. The control signal CS₀ is apulse width modulated signal. Control signal CS₀ waveform 114 representsan exemplary control signal CS₀. The controller 110 generates thecontrol signal CS₀ with two states as shown in the waveform 114. Eachpulse of control signal CS₀ turns switch 112 ON (i.e. conducts)represents a first state that causes the switch 112 to operateefficiently and minimize power dissipation by the switch 112. Duringeach pulse of control signal CS₀, the inductor current i_(L) increases,as shown in the exemplary inductor current waveform 115, to chargeinductor 116 during a charging phase T_(C). Diode 118 prevents currentflow from link capacitor 120 into switch 112. When the pulse of controlsignals CS₀ ends, the control signal CS₀ is in a second state, and theinductor 116 reverses voltage polarity (commonly referred to as“flyback”). The inductor current i_(L) decreases during the flybackphase T_(FB), as shown in inductor current waveform 115. The inductorcurrent i_(L) boosts the link voltage across the link capacitor 120through diode 118. When the flyback phase T_(FB) ends and when the nextcharging phase T_(C) begins depends on the operating mode of theswitching power converter. In discontinuous conduction mode (DCM), theflyback phase T_(FB) ends before the next charging phase T_(C) begins.However, regardless of whether the switching power converter 108operates in discontinuous conduction mode, continuous conduction mode,or critical conduction mode, the flyback phase T_(FB) begins as soon asthe charging phase T_(C) ends.

The switching power converter 108 is a boost-type converter, and, thus,the link voltage V_(LINK) is greater than the rectified input voltageV_(φR) _(_) _(IN). Controller 110 senses the rectified input voltageV_(φR) _(_) _(IN) at node 124 and senses the link voltage V_(LINK) atnode 126. Controller 110 operates the switching power converter 108 tomaintain an approximately constant link voltage V_(LINK) for lamp 122,provide power factor correction, and correlate the link current i_(LINK)with the phase cut angle of the rectified input voltage V_(φR) _(_)_(IN). Lamp 132 includes one or more light emitting diodes.

FIG. 3 depicts an exemplary light output/power graph 300 that compareslight output per watt of power for an exemplary incandescent bulb and anexemplary light emitting diode (LED). Per watt of power, LEDs providemore light output than incandescent light bulbs. The low power usage byLEDs correlates to a relatively low operating current compared to theoperating current for an incandescent light bulb. Since the light outputof LEDs is approximately linear with power and LEDs operate at anapproximately constant voltage, operating current for an LED decreasesapproximately linearly with decreasing light output and power.

Referring to FIGS. 1, 2, and 3, to decrease the light output of the lamp122, the phase cut dimmer 102 increases the phase cut angle of therectified input voltage V_(φR) _(_) _(IN), i.e. time T_(OFF) increasesand time T_(ON) decreases. The controller 110 responds to the increasedphase cut angle by decreasing the current i_(LINK) provided to the lamp122, which decreases the light output of the lamp 122.

The switching power converter 108 includes a power dissipation resistor128 so that the dimmer current i_(DIM) does not fall below the holdcurrent value and prematurely disconnect during a cycle of the rectifiedinput voltage V_(φR) _(_) _(IN). The “POWER IN” supplied to theswitching power converter 108 equals V_(φ) _(_) _(IN)·i_(DIM). The“POWER OUT” supplied by switching power converter 108 equalsV_(LINK)·i_(LINK). Because of the relatively low power requirements ofan LED based lamp 122, particularly at low light output levels, if thePOWER IN equals the POWER OUT+P_(INH), the dimmer current i_(DIM) mayfall below the hold current value and cause the phase-cut dimmer 102 toprematurely disconnect. In this situation, to prevent the dimmer currenti_(DIM) from falling below the hold current value, the controller 110causes the switching power converter 108 to maintain the dimmer currenti_(DIM) above the hold current value, which causes the POWER IN to begreater than the POWER OUT+P_(INH). Since the POWER IN is greater thanthe POWER OUT+P_(INH), the switching power converter 108 dissipates theexcess power through power dissipation resistor 128.

Because of component non-idealities, the switching power converter 108includes inherent power losses Inherent power losses include conductorresistances and switching losses in switch 112. However, circuits aregenerally designed to minimize inherent power losses, and these inherentpower losses are often negligible and, thus, insufficient to dissipateenough power to compensate for the difference between the POWER IN andthe POWER OUT+P_(INH) at some POWER OUT levels. To increase the powerloss of switching power converter so that the dimmer current i_(DIM)remains above a hold current value even at lower power demand by thelamp 122, switching power converter 108 includes the resistor 128 tocreate a passive power loss when switch 112 conducts the inductorcurrent _(L). For negligible inherent power losses, the resistance valueof the resistor 128 is selected so that when the switching powerconverter is providing a minimum link current i_(LINK), the POWERIN=POWER OUT+P_(INH)+PASSIVE POWER DISSIPATE.

Resistor 128 is relatively cheap to implement as part of switching powerconverter 108. However, when the link current i_(LINK) is sufficientlyhigh such that POWER IN equals POWER OUT+P_(INH), the dimmer inputcurrent i_(DIM) could be maintained above the hold current value withoutdissipating power through resistor 128. However, since the dimmer inputcurrent i_(DIM) always flows through the resistor 128 when the switch108 is conducts, the resistor 128 still passively dissipates powerregardless of whether the POWER IN is equal to the POWER OUT+P_(INH),which decreases the efficiency of lighting system 100.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, an apparatus includes anactive circuit to dissipate power in a link path of a switching powerconverter when the switching power converter receives more energy thanan amount of energy to be provided to a load coupled to the switchingpower converter.

In another embodiment of the present invention, a method includesactively dissipating power in a link path of a switching power converterwhen the switching power converter receives more energy than an amountof energy to be provided to a load coupled to the switching powerconverter.

In a further embodiment of the present invention, an apparatus includesmeans for actively dissipating power in a link path of a switching powerconverter when the switching power converter receives more energy thanan amount of energy to be provided to a load coupled to the switchingpower converter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features and advantages made apparent to those skilled in theart by referencing the accompanying drawings. The use of the samereference number throughout the several figures designates a like orsimilar element.

FIG. 1 (labeled prior art) depicts a lighting system that includes aleading edge dimmer.

FIG. 2 (labeled prior art) depicts exemplary voltage graphs associatedwith the lighting system of FIG. 1.

FIG. 3 (labeled prior art) depicts power versus light output forexemplary incandescent bulbs and light emitting diodes.

FIG. 4 depicts a lighting system that includes at least one or morepower dissipation circuits.

FIG. 5 depicts a lighting system that includes a switch path powerdissipation circuit.

FIG. 6 depicts an embodiment of the switch path power dissipationcircuit of FIG. 5.

FIGS. 7 and 8 depict exemplary waveforms present during an exemplaryoperation of the switch power dissipation circuit of FIG. 6.

FIG. 9 depicts another embodiment of the switch path power dissipationcircuit of FIG. 5.

FIG. 10 depicts a lighting system that includes a flyback path powerdissipation circuit.

FIGS. 11 and 12 depict respectively embodiments of the flyback pathpower dissipation circuit of FIG. 10.

FIGS. 13 and 14 depict exemplary waveforms present during an exemplaryoperation of the flyback path power dissipation circuits of FIGS. 11 and12.

FIG. 15 depicts another embodiment of the flyback path power dissipationcircuit of FIG. 10.

FIG. 16 depicts a lighting system that includes a link path powerdissipation circuit.

FIGS. 17 and 18 depict respective exemplary embodiments of the link pathpower dissipation circuit of FIG. 16.

FIG. 19 depicts an exemplary power dissipation phase interspersingtimeline.

FIG. 20 depicts an exemplary power dissipation intermixing andinterspersing timeline.

DETAILED DESCRIPTION

A lighting system includes one or more methods and systems to controldissipation of excess power in the lighting system when the power into aswitching power converter from a leading edge, phase-cut dimmer isgreater than the power out of the switching power converter. In at leastone embodiment, the lighting system includes a controller that controlsdissipation of excess energy in the lighting system to prevent apremature disconnection of the phase-cut dimmer. In at least oneembodiment, the controller actively controls power dissipation bygenerating one or more signals to actively and selectively control powerdissipation in the lighting system. By actively and selectivelycontrolling power dissipation in the lighting system, the controllerintentionally dissipates power when the power into the lighting systemshould be greater than the power out to a lamp of the lighting system.However, when the ‘power in’ can equal the ‘power out’ plus any inherentpower losses without causing the phase-cut dimmer to prematurelydisconnect, the controller causes the lighting system to operate moreefficiently by reducing or eliminating intentional power dissipation inthe lighting system.

To control dissipation of the excess energy, the controller controls oneor more power dissipation circuits during one or more controlled powerdissipation phases. In at least one embodiment, the controller createsone or more intermixed and/or interspersed power dissipation phases withone or more switching power converter charging and/or flyback phases.“Intermixed” refers to mixing one or more power dissipation phases withone or more charging and/or flyback phases. “Interspersed” refers toinserting one or more power dissipation phases between one or morecharging and/or flyback phases. The controlled power dissipation phaseoccurs after a charging phase begins and before an end of a subsequentflyback phase of the switching power converter. In at least oneembodiment, for a boost switching power converter, the charging phase isa phase when an inductor current of the switching power converter isincreasing and charging a boost inductor of the switching powerconverter. The flyback phase is when the inductor current decreases andboosts a link voltage of the switching power converter.

In at least one embodiment, the lighting system includes one, some, orall of a switch path, link path, and flyback path power dissipationcircuits to actively and selectively control power dissipation of excessenergy in a switching power converter of the lighting system. The switchpath power dissipation circuit dissipates power through a switch path inthe switching power converter of the lighting system. In at least oneembodiment, a controller is configured to control a boost switch in aswitching power converter of a phase cut compatible, dimmable lightingsystem. The controller is configured to control the boost switch in anefficient mode and a power dissipation mode. In the efficient mode, thecontroller is configured to operate the boost switch to minimize powerdissipation in the boost switch, and in the power dissipation mode thecontroller is configured to operate the boost switch to increasedissipation of energy in the boost switch relative to any powerdissipation in the boost switch during operation in the efficient mode.In at least one embodiment, the switch path includes a current source tolimit an inductor current through the boost switch. Limiting theinductor current through the boost switch causes the current sourceand/or the boost switch to dissipate power.

In at least one embodiment, the lighting system controls one or more ofthe timing, sequencing, and/or magnitude of the current through theboost switch, or any combination thereof, to control power dissipationby the lighting system. In at least one embodiment, controlling thetiming of the current refers to a duration of time in which the currentis limited or restricted. In at least one embodiment, controlling thesequencing of the current through the boost switch refers to selectingwhich charging and flyback phase time frames andor cycles of an inputvoltage to a switching power converter to control power dissipation inthe lighting system. In at least one embodiment, each charging andflyback time frame occurs between when a first charging phase followingan immediately preceding flyback phase begins and a flyback phaseimmediately preceding a next charging phase ends. In at least oneembodiment, the sequence of cycles is a consecutive series of cycles,and, in at least one embodiment, the sequence of time frames or cyclesis a non-consecutive series of time frames or cycles. In at least oneembodiment, controlling the magnitude of the current includescontrolling the internal resistance of the boost switch and/orcontrolling one or more current limits on the current through the boostswitch.

The flyback path power dissipation circuit dissipates power through aflyback path of the switching power converter. In at least oneembodiment, the lighting system controls power dissipation through aflyback path by controlling a transformer primary current in the flybackpath and, for example, limiting the primary current with a currentsource and dissipating power in the current source. In at least oneembodiment, the flyback path power dissipation circuit includes aflyback switch to limit the flyback current in the flyback switch. In atleast one embodiment, the flyback path includes a current source tolimit the flyback current. Limiting the flyback current through theflyback switch causes the current source and/or the flyback switch todissipate power. In at least one embodiment, the lighting systemcontrols one or more of the timing, sequencing, and/or magnitude of thecurrent through the flyback switch, or any combination thereof, tocontrol power dissipation by the lighting system.

The link path power dissipation circuit dissipates power through a linkpath of the switching power converter by controlling a link current ofthe switching power converter. In at least one embodiment, thecontroller controls the link path power dissipation circuit to limit thelink current with a current source and dissipating power in the currentsource. In at least one embodiment, the link path power dissipationcircuit includes an output switch to limit the link current bycontrolling an internal resistance of the switch. In at least oneembodiment, the link path includes a current source to limit the linkcurrent. Limiting the link current through the output switch causes thecurrent source and/or the output switch to dissipate power. In at leastone embodiment, the lighting system controls one or more of the timing,sequencing, and/or magnitude of the current through the output switch,or any combination thereof, to control power dissipation by the lightingsystem.

FIG. 4 depicts a lighting system 400 that includes at least one, some,or all of a controlled switch path power dissipation circuit 402, acontrolled link path power dissipation circuit 404, a controlled flybackpath power dissipation circuit 406, and a controller 408 to actively andselectively control power dissipation in a switching power converter 410of the lighting system 400. The power dissipation circuits 402, 404, and406 are shown in dotted lines because one or two of the powerdissipation circuits 402, 404, and 406 are optional. Whether to includeone, two, or three of the power dissipation circuits 402, 404, and 406and which power dissipation circuit(s) to include in lighting system 400is a matter of design choice. Including two or three of the powerdissipation circuits 402, 404, and 406 allows for distribution of powerdissipation among the included power dissipation circuits. In at leastone embodiment, power distribution is actively controlled by controller408. In at least one embodiment, power distribution is fixed orpreprogrammed. However, including more than one of the powerdistribution circuits 402, 404, and 406 can increase the complexity andcost of the switching power converter 410 and/or the complexity and costof the controller 408. Additionally, although the power distributioncircuits 402, 404, and 406 are shown as part of the switching powerconverter 410, in at least one embodiment, all or part of thedissipation circuits 402, 404, and 406 are located in controller 408.

As previously described, the phase-cut dimmer 102 can phase cut an inputvoltage V_(IN) supplied by voltage supply 104. The full-bridge dioderectifier 106 rectifies the phase cut input voltage V_(φ) _(_) _(IN) togenerate a rectified input voltage V_(φR) _(_) _(IN). In somecircumstances, especially at lower power output levels, to maintain thedimmer current i_(DIM) above a hold current value the switching powerconverter 410 draws more POWER IN from the voltage supply 104 than theP_(INH) plus the POWER OUT of the switching power converter 410.Assuming the inherent losses of the switching power converter 410 areinsufficient to dissipate enough power equal to a difference between thePOWER IN and the POWER OUT+P_(INH), the lighting system 400 controls oneor more of the power dissipation circuits 402, 404, and 406 so that thePOWER IN equals POWER OUT+P_(INH) plus power dissipated by one or moreof the power dissipation circuits 402, 404, and/or 406. The switchingpower converter 410 provides power to lamp 418. In at least oneembodiment, lamp 418 includes one or more light emitting diodes (LEDs),such as the series connected string of N LEDs 420. “N” represents apositive integer.

Controller 408 generates one or more respective control signals for eachof the dissipation circuits 402, 404, and 406 that are included in thelighting system 400. Control signals CS, CO, and CF respectively controlpower dissipation in the switch path power dissipation circuit 402, linkpath power dissipation circuit 404, and flyback path power dissipationcircuit 406. The switch path power dissipation circuit 402 dissipatespower through a switch path 412 in the switching power converter 410 ofthe lighting system 400 in accordance with the control signal CS. Thelink path power dissipation circuit 404 dissipates power through a linkpath 414 in the switching power converter 410 in accordance with thecontrol signal CO. The flyback path power dissipation circuit 406dissipates power through a flyback path 416 in the switching powerconverter 410 in accordance with the control signal CF. The particularmethod and circuit(s) used to implement the power dissipation circuits402, 404, and 406 and control dissipation of power through the switchpath 412 is a matter of design choice. Additionally, controlling thetiming, sequencing, and/or magnitude of power dissipation in the powerdissipation circuits 402, 404, and 406 is a matter of design choice.Exemplary embodiments of the power dissipation circuits 402, 404, and406 are subsequently described. The power dissipation circuits 402, 404,and 406 are depicted in FIG. 4 as wholly outside the controller 408.However, in at least one embodiment, all or part of one or more of thepower dissipation circuits 402, 404, and 406 are included within thecontroller 408.

The particular implementation of controller 408 is a matter of designchoice. For example, controller 408 can be (i) implemented as anintegrated circuit including, for example, a processor to executesoftware or firmware instructions stored in a memory, (ii) implementedusing discrete components, or (iii) implemented using any combination ofthe foregoing. In at least one embodiment, controller 408 generallyregulates the link voltage as described in U.S. patent application Ser.No. 11/967,269, entitled “Power Control System Using a NonlinearDelta-Sigma Modulator With Nonlinear Power Conversion Process Modeling”,filed on Dec. 31, 2007, inventor John L. Melanson (referred to herein as“Melanson I”), U.S. patent application Ser. No. 11/967,275, entitled“Programmable Power Control System”, filed on Dec. 31, 2007, andinventor John L. Melanson (referred to herein as “Melanson II”), U.S.patent application Ser. No. 12/495,457, entitled “Cascode ConfiguredSwitching Using at Least One Low Breakdown Voltage Internal, IntegratedCircuit Switch to Control At Least One High Breakdown Voltage ExternalSwitch”, filed on Jun. 30, 2009 (referred to herein as “Melanson III”),and inventor John L. Melanson, and U.S. patent application Ser. No.12,174,404, entitled “Constant Current Controller With Selectable Gain”,filing date Jun. 30, 2011, and inventors John L. Melanson, Rahul Singh,and Siddharth Maru, which are all incorporated by reference in theirentireties. The switching power converter 410 can be any type ofswitching power converter, such as a boost, buck, boost-buck, or Cúkswitching power converter. Switching power converter 410 includes othercomponents, such as an EMI capacitor, inductor, and link capacitor,which, for clarity of FIG. 4, are not shown but are subsequentlydescribed specific embodiments.

The manner of determining whether the POWER IN is greater than theP_(INH)+POWER OUT is a matter of design choice. In at least oneembodiment, the controller 408 includes the power monitor circuit 422.When power demand by the lamp 418 increases, the link voltage V_(LINK)decreases, which indicates an increase in the POWER OUT. Conversely,when power demand by the lamp 418 declines, the link voltage V_(LINK)increases, which indicates a decrease in the POWER OUT. The comparator424 of the power monitor circuit 422, thus, compares the link voltageV_(LINK) with a reference link voltage V_(LINK) _(_) _(REF). In at leastone embodiment, the reference link voltage V_(LINK) _(_) _(REF) is setto a voltage level that is a few volts or a few percent higher than thenominal voltage set for the lamp 418. If the link voltage V_(LINK)decreases below the reference link voltage V_(LINK) _(_) _(REF), theoutput P_(HIGH) of the comparator 424 is HIGH, which indicates anincrease in the POWER OUT. If the link voltage V_(LINK) increases abovethe reference link voltage V_(LINK) _(_) _(REF), the output P_(HIGH) ofthe comparator 424 is LOW, which indicates a decrease in the POWER OUT.In at least one embodiment, if normal operation of the switching powerconverter 410 does not prevent an increase of the link voltage V_(LINK)above the reference link voltage V_(LINK) _(_) _(REF), then the POWER INis greater than the POWER OUT+P_(INH), and controller 408 operates oneor more of the power dissipation circuits 412, 414, and 416 to dissipatethe excess energy represented by the difference between the POWER IN andthe POWER OUT+P_(INH).

FIG. 5 depicts a lighting system 500 that represents one embodiment oflighting system 400. Lighting system 500 includes a switch path powerdissipation circuit 502 to dissipate excess power in the lighting system500. The switch path power dissipation circuit 502 represents oneembodiment of the switch path power dissipation circuit 402. Theparticular implementation and control of the switch path powerdissipation circuit 502 is a matter of design choice. The switch pathpower dissipation circuit 502 includes a FET boost switch 504 in aboost-type switching power converter 508 and includes a controllablecurrent source 509. In at least one embodiment, the controller 506actively dissipates power in the switching power converter 508 during apower dissipation phase by limiting an inductor current i_(L) andthereby actively and selectively dissipates power in the lighting system500. Limiting the inductor current i_(L) through the boost switch 504causes the current source 509 and/or the boost switch 504 to dissipatethe excess power through switch path 511.

When the POWER IN is greater than the P_(INH)+POWER OUT to the load 518,the controller 506 controls the switch path power dissipation circuit502 to control dissipation of the excess energy by at least the boostswitch 504. Load 518 includes one or more LEDs. In at least oneembodiment, a gate voltage V_(G) biases a gate of boost switch 504 sothat controller 506 controls conductivity of the boost switch 504 usinga source control signal CS₁ as, for example, generally described inMelanson III. In other embodiments, controller 506 controls the gatevoltage V_(G) of boost switch 504 to control conductivity of the boostswitch 504 as, for example, generally described in Melanson I andMelanson II. Controller 506 represents one embodiment of controller 408.In at least one embodiment, the control signal CS₁ controls the value ofthe inductor current i_(L), as depicted by the exemplary, variableinductor current waveform 510.

The inductor current waveform 510 represents an exemplary inductorcurrent i_(L) waveform during controlled dissipation of energy throughthe boost switch 504. During a charging phase T_(C), the controller 506generates the control signal CS₁ to cause the boost switch 504 toconduct. When the boost switch 504 conducts, the inductor current i_(L)increases. When POWER IN is greater than POWER OUT+P_(INH), rather thanminimizing power loss, the controller 506 intentionally limits theinductor current i_(L), which causes dissipation of excess energy by atleast the boost switch 504 during a power dissipation phase T_(PD).Assuming that inherent losses in the switching power converter 508 arenegligible, the “excess energy” equals the POWER IN minus the (POWEROUT+P_(INH)). Limiting the inductor current i_(L) during the powerdissipation phase T_(PD) causes the change in the inductor currentdi_(L)/dt to move toward 0. Since the voltage V_(L) across the inductor116 equals L·di_(L)/dt, the voltage V_(L) is directly proportional tothe rate of change of the inductor current di_(L)/dt. “L” is theinductance of inductor 116. Thus, as the rate of change of the inductorcurrent di_(L)/dt moves toward 0, the rate of energy storage by theinductor 116 decreases toward 0 and more power is dissipated by theboost switch power dissipation circuit 502.

Referring to the control signal CS₁ waveform 513, in at least oneembodiment, the controller 506 is configured to control the boost switch504 in an efficient mode and a power dissipation mode. In the efficientmode, the controller 506 generates a two-state control signal CS₁, suchas the two-states of control signal CS₀ (FIG. 1), to operate the boostswitch 504 to minimize power dissipation in the boost switch 504. In thepower dissipation mode, the controller 506 is configured to operate theboost switch 504 to increase dissipation of energy in the boost switchrelative to any power dissipation in the boost switch 504 duringoperation in the efficient mode. In at least one embodiment, to operatethe boost switch 504 in the power dissipation mode, the controller 506generates the control signal CS₁ with at least three (3) states, such asstates “1”, “2”, and “3” in the waveform 513. During states 1 and 2, thecontroller 506 operates the boost switch 504 in an efficient mode tominimize power dissipation by the boost switch 504. During state 3, thecontroller 506 operates the boost switch 504 in a power dissipationmode. In state 3, the controller 506 limits the inductor current i_(L)and causes the control signal CS₁ to have a voltage that is greater thanstate “2” but lower than state “1”. Thus, the boost switch 504 does notturn OFF completely in state 3. State 3 is not simply a transient state,i.e. a continuous transition of control signal CS₁ between states 1 and2. State 3 is an intentional, non-transient state that alters powerdissipation by the boost switch 504. Thus, in state 3, the controller506 generates the control signal CS₁ by limiting the inductor currenti_(L) to cause a non-zero voltage V_(DS) across the boost switch 504 atthe same time that current i_(L) flows through the boost switch 504. Thesimultaneous occurrence of the voltage V_(DS) across the boost switch504 and the current i_(L) through the boost switch 504 causes powerdissipation by the boost switch 504. The number of states is a matter ofdesign choice and can be increased or decreased by, for example,controlling different limits of the inductor current i_(L).

For example, in at least one embodiment, the rate of change of theinductor current di_(L)/dt is driven by the controller 506 toapproximately 0. When the change di_(L)/dt in the inductor current i_(L)is 0, the inductor current i_(L) holds at a constant value, and thevoltage V_(L) across inductor 116 is approximately 0. During a chargingphase, the inductor current i_(L) increases. To dissipate power during acharging phase T_(C), the rate of change of the inductor currentdi_(L)/dt is decreased, which reduces the voltage V_(L) across theinductor 116. As the inductor voltage V_(L) decreases, the proportion ofpower dissipated by the switch path power dissipation circuit 502increases. During a flyback phase, the rate of change of the inductorcurrent di_(L)/dt and the inductor voltage V_(L) are negative. Thus, todissipate power during a flyback phase, the rate of change of theinductor current di_(L)/dt is increased towards 0, which increases theinductor voltage V_(L) toward 0 and increases the proportion of powerdissipated by the switch path power dissipation circuit 502.

In at least one embodiment, the current source 509 limits the inductorcurrent to an inductor current limit value i_(LIM). Thus, when a valueof the inductor current i_(L) through the boost switch 504 reaches theinductor current limit value i_(LIM), di_(L)/dt decreases with 0 or to asmaller value that a value that would otherwise occur without a powerdissipation phase T_(PD). The power dissipation phase T_(PD) occursafter the charging phase T_(C) and before the subsequent flyback phaseT_(FB) In at least one embodiment, the controller 506 intersperses thepower dissipation phase T_(PD) between the charging phase T_(C) and theflyback phase T_(FB) and causes the switch path power dissipationcircuit 502 to dissipate energy until the flyback period T_(FB) beginswhen the boost switch 504 is turned OFF.

In at least one embodiment, the inductor current limit value i_(LIM) iscontrollable by the controller 506 to adjust a duration of the powerdissipation period T_(PD). In at least one embodiment, the sourcecontrol signal CS₁ controls when the charging phase T_(C) and theflyback phase T_(FB) begin for each cycle of the rectified input voltageV_(φR) _(_) _(IN). In at least one embodiment, the power dissipationphase T_(PD) ends when the flyback phase T_(FB) begins. Thus, sincecontroller 506 generates the source control signal CS₁, controller 506controls the duration of the power dissipation phase T_(PD) bycontrolling when to begin the flyback phase T_(FB).

The controller 506 controls interspersing and/or intermixing of one ormore power dissipation phases with one or more charging and/or flybackphases. In at least one embodiment, the controller 506 intersperses apower dissipation phase T_(PD) between charging phases or a flybackphase by reducing the change in the inductor current i_(L) over time,i.e. di_(L)/dt, by reducing di_(L)/dt to zero. When di_(L)/dt is reducedto zero, power dissipation occurs through the boost switch 504. In atleast one embodiment, the controller intermixes a power dissipationphase T_(PD) with a charging phase T_(C) or a flyback phase T_(FB) byreducing di_(L)/dt to a non-zero value. When di_(L)/dt is reduced to anon-zero value, the charging phase T_(C) or flyback phase T_(FB)continues in combination with power dissipation by the switch path powerdissipation circuit 502 through the boost switch 504.

FIG. 6 depicts a switch path power dissipation circuit 600, whichrepresents one embodiment of the switch path power dissipation circuit502. FIG. 7 depicts exemplary waveforms 700 for an exemplary inductorcurrent i_(L), boost switch 504 drain-to-source voltage V_(DS), andcontrol signal CS₁ present during an exemplary operation of the switchpower dissipation circuit 600. Referring to FIGS. 5, 6, and 7, theswitch path power dissipation circuit 600 includes a controllablecurrent source 602, which represents one embodiment of the controllablecurrent source 509. The current source 602 includes FETs 604 and 606,which are configured as a current mirror. In at least one embodiment,the controller 506 modulates the control signal CS₁ to control currentthrough switch 504 using at least three (3) states. States 1 and 2 areefficient states when power dissipation by the boost switch 504 isminimized. State 3 is an inefficient or power dissipation state whencontroller 506 intentionally and actively causes the boost switch 504 todissipate power.

When controller 506 causes the source control signal CS₁ to become alogical 0, the boost switch 504 turns ON, and the inductor current i_(L)begins to ramp up at the beginning of charging period T_(C) _(_) ₀. Whenthe inductor current i_(L) is ramping up, the control signal CS₁ is instate 2, which allows the boost switch 504 to operate efficiently, i.e.minimize internal power loss by the boost switch 504. In at least oneembodiment, the boost switch 504 turns ON at state 2, and the inductori_(L) flows through boost switch 504 and FET 604. Current source 608supplies a reference current i_(REF), which flows through FET 606. In atleast one embodiment, control signal CS₁ turns boost switch 504 ON instate 2 with sufficient voltage to minimize the internal resistance ofboost switch 504. The size of FET 604 is scaled to the size of FET 606by a scaling factor of Z. The value of the scaling factor Z is apositive number and a matter of design choice. The value of the scalingfactor Z times the value of the reference current i_(REF) sets aninductor current limit value i_(LIM). Thus, when the inductor currenti_(L) reaches the inductor current limit value i_(LIM), the inductorcurrent i_(L) will stop increasing. In at least one embodiment, when theinductor current i_(L) reaches the inductor current limit value i_(LIM),the charging phase T_(C) _(_) ₀ ends and a power dissipation phaseT_(PD) _(_) ₀ begins. At the current limit i_(LIM), the control signalCS₁ is at the non-transient state 3, and power is dissipated by theswitch 504. Once the inductor current i_(L) reaches the current limitvalue i_(LIM), the inductor current i_(L) becomes a constant equal toi_(LIM), and boost switch 504 and FET 604 dissipate the excess energy inthe switching power converter 508.

When the boost switch 504 turns OFF, the power dissipation phase T_(PD)_(_) ₀ ends, and the flyback phase T_(FB) _(_) ₀ begins. Indiscontinuous conduction mode (DCM) and critical conduction mode (CRM),the flyback phase T_(FB) _(_) ₀ continues until the inductor currenti_(L) reaches zero. In continuous conduction mode (CCM), the nextcharging phase T_(C) _(_) ₁ begins prior to the inductor current i_(L)reaching zero. The waveforms 700 illustrate the switching powerconverter 508 operating in DCM. The switching power converter 508 canalso operate in CCM and CRM. In at least one embodiment, when operatingin DCM, once the link voltage V_(L) drops to a predetermined value, thecontroller 506 generates control signal CS₁ to cause the boost switch504 to conduct and initiate the next charging phase T_(C) _(_) ₁. Whenthe inductor current i_(L) reaches the inductor current limit valuei_(LIM), the next power dissipation phase T_(PD) _(_) ₁ begins and soon.

The duration of the power dissipation phases T_(PD) _(_) ₀, T_(PD) _(_)₁, and so on is controlled by the controller 506 and are a matter ofdesign choice. In at least one embodiment, the duration of the powerdissipation phases is sufficient to dissipate all excess energy in asingle cycle of the rectified input voltage V_(φR) _(_) _(IN). In atleast one embodiment, the duration of the power dissipation phases isvaried and sequenced to dissipate all excess energy in consecutive ornon-consecutive cycles of the rectified input voltage V_(φR) _(_) _(IN).

Additionally, in at least one embodiment, current source 608 can varythe value of the reference current i_(REF) in accordance with anoptional current reference control signal C_(iREF) generated bycontroller 506. Varying the value of the reference current i_(REF) alsovaries the inductor limit current i_(LIM) in accordance with the scalingfactor Z. By varying the reference current i_(REF) and, thus, theinductor limit current i_(LIM) during a single cycle of the rectifiedinput voltage V_(φR) _(_) _(IN), the controller 506 can stage powerdissipation. The controller 506 can also vary the inductor limit currenti_(LIM) during consecutive or non-consecutive cycles of rectified inputvoltage V_(φR) _(_) _(IN) to manage power dissipation in switching powerconverter 508.

Additionally, in at least one embodiment, current source 608 can varythe value of the scaling factor Z in accordance with an optional scalingfactor control signal C_(SCALE) generated by controller 506. Varying thescaling factor Z also varies the inductor limit current i_(LIM) inaccordance with the scaling factor Z. By varying the scaling factor Zand, thus, the inductor limit current i_(LIM) during a single cycle ofthe rectified input voltage V_(φR) _(_) _(IN), the controller 506 canstage power dissipation. The controller 506 can also vary the inductorlimit current i_(LIM) during consecutive or non-consecutive cycles ofrectified input voltage V_(φR) _(_) _(IN) to manage power dissipation inswitching power converter 508. In at least one embodiment, the FETs 604and/or 606 are implemented using multiple, parallel connected FETs (notshown). In at least one embodiment, the scaling factor control signalC_(SCALE) changes the number of FETs used to implement FETs 604 and/or606 and, thus, changes the scaling factor. For example, in at least oneembodiment, one FET is used to implement FET 606 and 200 FETs identicalto the one FET used to implement FET 606 are used to implement FET 604,which provides a scaling factor of 200. By disabling one or more of theFETs used to implement FET 604, the controller varies the scaling factorZ. Additionally, in at least one embodiment, the controller 506 controlsboth the reference current i_(REF) and the scaling factor C_(SCALE) tocontrol the inductor current i_(L).

FIG. 8 depicts an exemplary inductor current i_(L) and control signalCS₁ waveforms 800 for when the controller 506 causes the switch pathpower dissipation circuit 502 to dissipate excess power. Referring toFIGS. 5, 6, and 8, charging phases and feedback phases can beinterspersed and intermixed with power dissipation phases as desired tocontrol the timing, sequencing, and magnitude of power dissipation inthe boost switch 504. The particular timing and amount of intermixingand interspersing power dissipation in the switch path power dissipationcircuit 600 is a matter of design choice. The waveforms 800 represent anexemplary choice.

In at least one embodiment, the controller 506 modulates the controlsignal CS₁ to control current through switch 504 using at least four (4)states. States 1 and 2 are efficient states when the controller 506operates the boost switch 504 in an efficient mode and, thus, minimizespower dissipation by the boost switch 504. States 3 and 4 areinefficient states when the controller 506 operates the boost switch 504in a power dissipation mode. During states 3 and 4 in the powerdissipation mode, the controller 506 intentionally and actively causesthe boost switch 504 to dissipate power.

Referring to the waveforms 800 and the switch path power dissipationcircuit 600, during the charging phase T_(C) _(_) ₁, the control signalCS₁ causes the boost switch 504 to saturate and the inductor currenti_(L) increases over time. At the beginning of the intermixed chargingphase T_(C) _(_) ₂ and power dissipation phase T_(PD) _(_) ₁, thecontroller 506 generates the current reference control signal C_(IREF)and/or the scaling control signal C_(SCALE) to decrease the rate ofincrease of the inductor current i_(L), i.e. decrease di_(L)/dt, and thecontrol signal CS₁ is in state 3.

At the beginning of the third charging phase T_(C) _(_) ₃, thecontroller 506 reenters an efficient mode for boost switch 504 andincreases the inductor current i_(L) rate of change di_(L)/dt, whichreduces power dissipation in the boost switch 504. In state 3, thecontroller 504 causes the boost switch 504 to operate in a powerdissipation mode. The controller 506 intersperses the second powerdissipation phase T_(PD) _(_) ₂ between the intermixed second chargingphase T_(C2) and first power dissipation phase T_(PD) _(_) ₁ by limitingthe rate of change di_(L)/dt of the inductor current i_(L) to 0.Limiting di_(L)/dt to 0 holds the inductor current i_(L) constant anddissipates excess power through the boost switch 504 and in the currentsource 602. Controller 506 generates a control signal CS₁ that weaklyleaves the boost switch 504 ON but allows the flyback phase T_(FB) _(_)₁ to be intermixed with a third power dissipation phase T_(PD) _(_) ₃.During the interspersed power dissipation phase T_(PD) _(_) ₄,controller 506 turns the boost switch 504 OFF, and controller 506 causesthe inductor current i_(L) to be limited and di_(L)/dt to equal zero.During the second feedback phase T_(FB) _(_) ₂, the controller 502 turnsthe boost switch 504 ON to allow the full inductor current i_(L) tocharge the link capacitor 120.

During power dissipation phase T_(PD) _(_) ₄, the controller 506operates the boost switch 504 in a power dissipation mode by generatingthe control signal CS₁ in state 4. State 4 corresponds to a limiting ofthe inductor current i_(L) at a lower limit than the limit associatedwith state 3.

FIG. 9 depicts switch path power dissipation circuit 900, whichrepresents one embodiment of the switch path power dissipation circuit502. Operational amplifier 902 provides a feedback path to control thegate voltage V_(G) and source voltage V_(S) of boost switch 504. Thecontroller 506 controls the reference voltage V_(REF), and thecomparator 902 drives the gate voltage V_(G) so that the source voltageV_(S) of the boost switch 504 equals the reference voltage V_(REF). Thesource voltage V_(S) and the voltage level of the control signal CS₁create a voltage difference across the power dissipation resistor 904,which sets the value of the inductor current i_(L). The inductor currenti_(L) flows through the boost switch 504 and the power dissipationresistor 904. Since the controller 506 controls the source voltage V_(S)and the voltage level of the control signal CS₁, controller 506 controlsthe value of the inductor current i_(L). Thus, the controller 506 cancontrol the inductor current i_(L) and intermix and/or intersperse powerdissipation phases as described in conjunction, for example, with FIGS.7 and 8. The particular timing and amount of intermixing andinterspersing of power dissipation in the switch path power dissipationcircuits 502, 600, and 900 is a matter of design choice.

FIG. 10 depicts a lighting system 1000, which represents one embodimentof the lighting system 400. The lighting system 1000 includes acontrolled flyback path power dissipation circuit 1002, which representsone embodiment of the controlled flyback path power dissipation circuit406. In at least one embodiment, the lighting system 1000 controls powerdissipation through a flyback path 1004 by controlling a transformerprimary current i_(P) in the flyback path 1002 and limiting the primarycurrent i_(P) to control power dissipation.

The lighting system 1000 also includes a controller 506 that controlsthe flyback path power dissipation circuit 1002 and generates controlsignal CS₂ to control switching power converter 1008. In at least oneembodiment, switching power converter 1008 is a boost-type switchingpower converter, such as switching power converter 108 (FIG. 1), andcontroller 506 controls the switching power converter 1008 as, forexample, generally described in Melanson I and Melanson II.

In at least one embodiment, the flyback path power dissipation circuit1002 modulates the primary current i_(P) to energize the primary-sidecoil 1010 of transformer 1012. Transformer 1012 transfers energy fromthe primary-side coil 1010 to the secondary-side coil 1014 to cause asecondary current i_(S) to flow through diode 1016 and charge loadvoltage capacitor 1018 to the load voltage V_(LD). The load voltageV_(LD) provides a voltage across lamp 1020.

When the POWER IN is greater than the POWER OUT+P_(INH), controller 506operates the flyback path power dissipation circuit 1002 to dissipateexcess energy. The particular implementation and operation of theflyback path power dissipation circuit 1002 to dissipate the excessenergy is a matter of design choice.

FIG. 11 depicts flyback dissipation circuit 1100, which represents oneembodiment of the flyback power dissipation circuit 1002. Flybackdissipation circuit 1100 includes controllable current source 1102 tocontrol the primary current i_(P) through the flyback FET 1104.Controller 506 generates one or more current source control signals CSCSand control signal CS₂ to control the primary current through flybackswitch 1104. Controlling the primary current i_(P) allows the flybackdissipation circuit 1100 to control power dissipation in a mannersimilar to the control of power dissipation in the switch path powerdissipation circuit 502 (FIG. 5).

FIG. 12 depicts flyback path power dissipation circuit 1200, whichrepresents one embodiment of flyback path power dissipation circuit1002. In at least one embodiment, controller 506 controls powerdissipation through the flyback path 1004 by controlling the transformerprimary current i_(P) and, for example, limiting the primary currenti_(P) with a current source 1202 and dissipating power in the currentsource 1202. In at least one embodiment, the current source 1202 isidentical to the current source 602 (FIG. 6) and functions as describedin conjunction with current source 602. In at least one embodiment, thecurrent source 1202 limits the primary current i_(P) through the flybackFET 1104 to limit the primary current i_(P) (also referred to as a“flyback current”). Limiting the primary current i_(P) through theflyback switch 1104 causes the current source 1202 to dissipate power.In at least one embodiment, the controller 506 controls interspersing,intermixing, and sequencing of power dissipation through the flybackswitch 1104 and the current source 1202 to control power dissipation bythe lighting system 1000 (FIG. 10). As with the lighting system 500, inat least one embodiment, the controller 506 generates the control signalCS₂ to operate flyback switch 1104 in an efficient mode when notdissipating power by the flyback switch 1104. Also, as with lightingsystem 500, the controller 506 generates the control signal CS₂ bylimiting the primary current i_(P) to operate the flyback switch 1104 inthe power dissipation mode. Thus, the controller 506 generates thecontrol signal CS₂ with at least three (3) states. The number of statesis a matter of design choice and can be increased or decreased, forexample, by controlling different limits of the primary current i_(P).

FIG. 13 depicts waveforms 1302 and 1304, which depict exemplaryprimary-side and secondary-side currents for the flyback powerdissipation circuits 1000 and 1100. Referring to FIGS. 12 and 13,current i_(P) _(_) ₀ and i_(S) _(_) ₀ in waveforms 1302 representrespective primary-side and secondary-side currents. When there is noactive control of power dissipation by controller 506 in the flybackpath power dissipation circuit 1200, flyback switch 1104 is ON duringthe charging phase T_(C) during which primary-side current i_(P) _(_) ₀ramps up. When controller 506 turns the flyback switch 1104 OFF, theflyback phase T_(FB) _(_) ₀ begins. As with the lighting system 500, inat least one embodiment, the controller 506 generates the control signalCS₂ to operate flyback switch 1104 in an efficient mode when notdissipating power by the flyback switch 1104.

Referring to waveforms 1304, when controller 506 actively controls powerdissipation in the flyback path power dissipation circuit 1200, in atleast one embodiment, the current source 1202 generates the primary-sidelimit current i_(LIM) _(_) _(FB) to limit the primary-side current i_(P)and delay the flyback phase T_(FB) _(_) ₁ until completion of the powerdissipation phase T_(PD). During the power dissipation phase T_(PD), theprimary-side current i_(P) is constant, so the voltage across theprimary-side coil 1010 is zero, and power is dissipated through theflyback FET 1104 and the current source 1202.

FIG. 14 depicts waveforms 1400, which depict exemplary primary-side andsecondary-side currents for the flyback power dissipation circuits 1000and 1100. In at least one embodiment, the controller 506 coordinatemultiple primary-side limit currents, such as limit currents i_(LIM)_(_) _(FB) _(_) ₁ and i_(LIM) _(_) _(FB) _(_) ₂ to stage the powerdissipation by the flyback path power dissipation circuit 1200.Controller 506 can control the multiple limit currents i_(LIM) _(_)_(FB) _(_) ₁ and i_(LIM) _(_) _(FB) _(_) ₂ by setting the referencecurrent i_(REF) with the controls signal C_(IREF), setting the scalingfactor Z with the control signal C_(SCALE), or setting both thereference current i_(REF) and the scaling factor Z, as described inconjunction with the current source 602 (FIG. 6). Staging the powerdissipation by the flyback power dissipation circuit 1200 to, forexample, thermally manage power dissipation by the flyback switch 1104and the current source 1202.

FIG. 15 depicts a flyback path power dissipation circuit 1500, whichrepresents one embodiment of the flyback path power dissipation circuit1002. In at least one embodiment, the flyback path power dissipationcircuit 1500 dissipates power through a resistor 1502. An auxiliarypower winding 1504 receives energy from the primary-side coil 1210during a flyback phase of the primary-side coil. A gate voltage V_(G)biases a gate of FET 1506, and controller 506 controls the conductivityof FET 1506 with source control signal CS₃. When the controller 506determines that POWER IN does not equal POWER OUT+P_(INH), thecontroller 506 turns FET 1506 ON, which allows current i_(AUX) from theauxiliary winding 1504 to flow through diode 1508 and through resistor1502. In at least one embodiment, the controller 506 dissipates anamount of power equal to a difference between POWER IN and POWEROUT+P_(INH) over one or more cycles of the auxiliary current i_(AUX).

FIG. 16 depicts a lighting system 1600 that represents one embodiment oflighting system 400. Lighting system 1600 includes a link path powerdissipation circuit 1602 to dissipate excess power in the lightingsystem 1600. The link path power dissipation circuit 1602 is an activecircuit that represents one embodiment of the link path powerdissipation circuit 404. In general, when the POWER IN exceeds the POWEROUT+P_(INH), the link path power dissipation circuit 1602 activelydissipates excess energy through the output power dissipation path 1604.An active circuit includes circuit components other than passivecomponents, such as resistors. An active circuit is responsive to one ormore signals, such as a link voltage signal. In at least one embodiment,the link path power dissipation circuit 1602 monitors the link voltageV_(LINK). When the POWER IN exceeds the POWER OUT+P_(INH), the linkvoltage V_(LINK) will increase if excess energy is not dissipated by thelighting system 1600. Thus, in at least one embodiment, the link pathpower dissipation circuit 1602 monitors the link voltage V_(LINK), andwhen the link voltage V_(LINK) exceeds a predetermined reference linkvoltage, the link path power dissipation circuit 1602 dissipates theexcess energy. The particular implementation and control of the linkpath power dissipation circuit 1602 is a matter of design choice.

FIG. 17 depicts an exemplary link path power dissipation circuit 1700,which represents one embodiment of the link path power dissipationcircuit 1602. The link path power dissipation circuit 1700 includesseries connected resistors 1702 and 1704 that form a resistor-basedvoltage divider to generate a scaled link voltage V_(LINK) _(_)_(SCALE). The link path power dissipation circuit 1700 monitors the linkvoltage V_(LINK) by comparing the scaled link voltage V_(LINK) _(_)_(SCALE) with a reference link voltage V_(LINK) _(_) _(REF). Thereference link voltage V_(LINK) _(_) _(REF) establishes a threshold forpower dissipation by the link path power dissipation circuit 1700. Thescaled link voltage V_(LINK) _(_) _(SCALE) biases the non-invertinginput terminal of the comparator 1706, and the reference link voltageV_(LINK) _(_) _(REF) biases the inverting input terminal of thecomparator 1706. When the scaled link voltage V_(LINK) _(_) _(SCALE)exceeds the reference link voltage V_(LINK) _(_) _(REF), the comparator1706 biases a switch 1708, which causes the primary current i_(P) toflow through power dissipation resistor 1710 and switch 1708. Theimplementation of switch 1708 is a matter of design choice. In at leastone embodiment, switch 1708 is a bipolar junction transistor (BJT), andthe comparator 1706 biases a base of the BJT switch 1708. In at leastone embodiment, switch 1708 is FET, and the comparator 1706 biases agate of the FET switch 1708. Reciprocally, when the scaled link voltageV_(LINK) _(_) _(SCALE) is less than the reference link voltage V_(LINK)_(_) _(REF), the comparator 1706 turns the switch 1708 OFF, which stopscurrent flow in and power dissipation by the power dissipation resistor1710. The particular link voltage V_(LINK) corresponding to thereference link voltage V_(LINK) _(_) _(REF) is a matter of design choiceand is, for example, 105%-120% of a normal operating link voltageV_(LINK). In at least one embodiment, the reference link voltageV_(LINK) _(_) _(REF) is approximately 115% for an input voltage V_(IN)equal to 110 Vrms and 107% for an input voltage V_(IN) equal to 230Vrms.

FIG. 18 depicts an exemplary link path power dissipation circuit 1800,which represents one embodiment of the link path power dissipationcircuit 1602. The link path power dissipation circuit 1800 includesseries connected resistors 1802 and 1804 that form a resistor-basedvoltage divider to generate a scaled link voltage V_(LINK) _(_)_(SCALE). The link path power dissipation circuit 1800 monitors the linkvoltage V_(LINK) by using the analog-to-digital converter 1806 toconvert the analog scaled link voltage V_(LINK) _(_) _(SCALE) into adigital value scaled link voltage V_(LINK) _(_) _(SCALE)(n). Logic 1808determines if the link path power dissipation circuit 1800 shoulddissipate excess energy by determining if the scaled link voltageV_(LINK) _(_) _(SCALE) indicates that the link voltage V_(LINK) isgreater than a particular threshold value. The particular thresholdvalue is a matter of design choice and is, for example, 105%-120% of anormal operating link voltage V_(LINK). If the logic 1808 determinesthat the link path power dissipation circuit 1800 should dissipateexcess energy, the logic 1808 controls conductivity of switch 1812. Inat least one embodiment, switch 1812 is a BJT, and the logic 1808controls the current source 1812 to bias an emitter of the BJT switch1812 and control flow of the primary current i_(P) through the powerdissipation resistor 1814. In at least one embodiment, switch 1812 is aFET, and the logic 1808 controls the current source 1812 to bias asource of the FET switch 1812 and control flow of the primary currenti_(P) through the power dissipation resistor 1814.

The implementation of the logic 1808 and current source 1810 is a matterof design choice. In at least one embodiment, the current source 1810 isidentical to the current source 602 (FIG. 6), and the logic 1808 cancontrol the reference current (not shown) and/or the scaling factor (notshown) of the current source 1810. In at least one embodiment, the logic1808 comprises a processor (not shown) that executes code to determinethe particular intermixing, interspersing, and limits for the primarycurrent during power dissipation in accordance with a predeterminedalgorithm. In at least one embodiment, the algorithm is stored asexecutable code in a memory (not shown) of the logic 1808. Theparticular algorithm is a matter of design choice. In at least oneembodiment, the algorithm causes the power dissipation resistor 1814 todissipate power until the scaled link voltage V_(LINK) _(_) _(SCALE)indicates that the link voltage V_(LINK) has declined to a predeterminedvalue, such as a normally operating level for the lamp 418 (FIGS. 4 and16).

Referring to FIG. 4, in at least one embodiment, controller 408 controlsthe switch path power dissipation circuit 402, the link path powerdissipation circuit 404, and/or the flyback path power dissipationcircuit 406 to dissipate power when the POWER IN is greater than thePOWER OUT+P_(INH) plus the inherent losses of the lighting system 400.In at least one embodiment, the controller 408 can introduce powerdissipation phases as needed to dissipate excess energy. FIG. 19 depictsan exemplary power dissipation phase interspersing timeline 1900 forthree exemplary time frames A, B, and C. In at least one embodiment, asingle timeframe, such as time frames A, B, or C, refers to a timebetween when a first charging phase following an immediately precedingflyback phase begins and a flyback phase immediately preceding a nextcharging phase ends. In time frame A, the power dissipation phase 1902is interspersed between charging phase 1904 and flyback phase 1906. In asubsequent time frame B of the rectified input voltage V_(φR) _(_)_(IN), the flyback phase 1908 immediately follows the charging phase1910, and, there is no power dissipation phase in time frame B. Timeframe B can be a consecutive time frame after time frame A or anon-consecutive time frame. A controller 506 (FIG. 5) can avoidincluding a power dissipation phase in time frame B for a variety ofreasons, such as when the POWER IN equals the POWER OUT+P_(INH) or toallow components to cool prior to initiating another power dissipationphase. In time frame C of the rectified input voltage V_(φR) _(_) _(IN),controller 506 intersperse the power dissipation phase 1912 betweencharging phase 1914 and flyback phase 1916.

FIG. 20 depicts an exemplary power dissipation intermixing andinterspersing timeline 2000 for a single timeframe. In at least oneembodiment, a single timeframe refers to a time between when a firstcharging phase following an immediately preceding flyback phase beginsand a flyback phase immediately preceding a next charging phase ends.The power dissipation phase 2002 is intermixed with the charging phase2004 and interspersed with a subsequent charging phase 2006. Asubsequent power dissipation phase 2008 occurs after the charging phase2006. The power dissipation phase 2010 is interspersed between flybackphases 2012 and 2014 and also intermixed with flyback phase 2014. Thepower dissipation phase 2009 begins after the beginning of the chargingphase 2006 and before an end of the subsequent flyback phase 2012. Thenumber and timing of interspersed and intermixed charging and flybackphases and power dissipation phases is a matter of design choice anddepends on, for example, an amount of power to be dissipated and thermalmanagement of components.

Thus, a lighting system includes one or more methods and systems tocontrol dissipation of excess power in the lighting system when thepower into a switching power converter from a leading edge, phase-cutdimmer is greater than the power out of the switching power converter.In at least one embodiment, to control dissipation of the excess energy,the controller controls one or more power dissipation circuits duringone or more controlled power dissipation phases. In at least oneembodiment, the controller creates one or more intermixed and/orinterspersed power dissipation phases with one or more switching powerconverter charging and/or flyback phases.

Although embodiments have been described in detail, it should beunderstood that various changes, substitutions, and alterations can bemade hereto without departing from the spirit and scope of the inventionas defined by the appended claims.

What is claimed is:
 1. An apparatus comprising: an active circuit todissipate excess power in a link path of a switching power converterwhen the switching power converter receives more energy during a periodof time than an amount of energy to be provided to a load coupled to theswitching power converter plus inherent energy losses, wherein theexcess power comprises power received by the switching power converterto prevent a premature disconnection of a phase-cut dimmer.
 2. Theapparatus of claim 1 wherein the active circuit comprises: a resistorcoupled to a link capacitor of the switching power converter; and acontrol circuit to dissipate energy through the resistor when thecontrol circuit determines the switching power converter receives moreenergy than an amount of energy to be provided to a load coupled to theswitching power converter plus inherent energy losses of the switchingpower converter.
 3. The apparatus of claim 2 wherein the control circuitcomprises: a switch coupled to the resistor; a comparator having anoutput terminal coupled to the coupled to the switch, a first inputterminal to sample a link voltage of the switching power converter, anda second input terminal to receive a reference voltage, wherein thecomparator is configured to cause the switch to conduct when the linkvoltage is greater than the reference voltage and, when the switchconducts, energy in the link path of the switching power converter isdissipated through the resistor.
 4. The apparatus of claim 2 wherein thecontrol circuit comprises: a switch coupled to the resistor; wherein thecontrol circuit is configured to turn the switch ‘on’ and ‘off’ todissipate the power in the link path of the switching power converterwhen the switching power converter receives more energy than an amountof energy to be provided to a load coupled to the switching powerconverter plus inherent power losses in the switching power converter.5. The apparatus of claim 4 wherein the control circuit is configured tocontrol a duty cycle of the switch to control the dissipation of power.6. The apparatus of claim 1 wherein the active circuit comprises: aresistor coupled to a link capacitor of the switching power converter; aswitch coupled to the resistor; and a control circuit to dissipateenergy through the resistor, the switch, or through both the resistorand the switch when the control circuit determines the switching powerconverter receives more energy than an amount of energy to be providedto a load coupled to the switching power converter plus inherent energylosses by the switching power converter.
 7. The apparatus of claim 1wherein the active circuit comprises: a degenerated, series coupled setof resistors coupled to a link capacitor of the switching powerconverter; and a control circuit to select a subset of the resistors todissipate energy with the selected subset of the resistors when thecontrol circuit determines the switching power converter receives moreenergy than an amount of energy to be provided to a load coupled to theswitching power converter.
 8. The apparatus of claim 1 wherein theactive circuit comprises a device that draws a current while maintainingan approximately constant voltage.
 9. The apparatus of claim 1 whereinthe amount of energy to be provided to the load is an amount of energyused by a light emitting diode emulating a brightness output of anincandescent light.
 10. A method comprising: receiving excess power in aswitching power converter, wherein the excess power comprises powerreceived by the switching power converter to prevent a prematuredisconnection of a phase-cut dimmer; and actively dissipating the excesspower in a link path of a switching power converter when the switchingpower converter receives more energy than an amount of energy to beprovided to a load coupled to the switching power converter plusinherent energy losses.
 11. The method of claim 10 wherein the activecircuit comprises: determining when the switching power converterreceives more energy than an amount of energy to be provided to a loadcoupled to the switching power converter plus inherent power losses inthe switching power converter; and controlling dissipation of the energythrough a resistor.
 12. The method of claim 11 wherein: determining whenthe switching power converter receives more energy than an amount ofenergy to be provided to a load coupled to the switching power converterplus inherent power losses in the switching power converter furthercomprises: sampling a link voltage of the switching power converter;receiving a reference voltage; and comparing the link voltage and thereference voltage; and controlling dissipation of the energy through aresistor further comprises: causing a switch to conduct when the linkvoltage is greater than the reference voltage and, when the switchconducts, energy in the link path of the switching power converter isdissipated through the resistor.
 13. The method of claim 11 whereinactively dissipating power in a link path of a switching power converterfurther comprises: turning a switch ‘on’ and ‘off’ to dissipate thepower in the link path of the switching power converter when theswitching power converter receives more energy than an amount of energyto be provided to a load coupled to the switching power converter plusinherent power losses in the switching power converter.
 14. The methodof claim 13 further comprising: controlling a duty cycle of the switchto control the dissipation of power.
 15. The method of claim 10 whereinthe active circuit comprises: determining when the switching powerconverter receives more energy than an amount of energy to be providedto a load coupled to the switching power converter plus inherent energylosses by the switching power converter; and wherein activelydissipating power in a link path of the switching power convertercomprises dissipating energy through a resistor, a switch coupled to theresistor, or through both the resistor and the switch when the switchingpower converter receives more energy than an amount of energy to beprovided to a load coupled to the switching power converter plusinherent energy losses by the switching power converter.
 16. The methodof claim 10 wherein the active circuit comprises: determining when theswitching power converter receives more energy than an amount of energyto be provided to a load coupled to the switching power converter plusinherent energy losses by the switching power converter; and selecting asubset of a degenerated, series coupled set of resistors coupled to alink capacitor of the switching power converter to dissipate energy withthe selected subset of the resistors when the switching power converterreceives more energy than an amount of energy to be provided to a loadcoupled to the switching power converter.
 17. The method of claim 10wherein the amount of energy to be provided to the load is an amount ofenergy used by a light emitting diode emulating a brightness output ofan incandescent light.
 18. An apparatus comprising: a load; a switchingpower converter coupled to the load; a controller coupled to theswitching power converter, wherein the controller is configured to causethe switching power converter to receive excess power when an amount ofpower to be provided to the load plus inherent energy losses in theswitching power converter is less than an amount of power received bythe switching power converter that prevents a premature disconnection ofa phase-cut dimmer; and an active circuit to dissipate the excess powerin a link path of the switching power converter.
 19. The apparatus ofclaim 18 further comprising: means for determining when the switchingpower converter receives more energy than an amount of energy to beprovided to a load coupled to the switching power converter plusinherent power losses in the switching power converter; and means forcontrolling dissipation of the energy through a resistor.
 20. Theapparatus of claim 1 wherein the active circuit is further configured todissipate the excess energy only when the switching power converterreceives more energy than an amount of energy to be provided to a loadcoupled to the switching power converter plus inherent energy losses.21. The method of claim 10 wherein actively dissipating excess power ina link path of a switching power converter occurs only when theswitching power converter receives more energy than an amount of energyto be provided to a load coupled to the switching power converter plusinherent energy losses.
 22. The apparatus of claim 18 wherein the activecircuit is further configured to dissipate the excess energy only whenthe switching power converter receives more energy than an amount ofenergy to be provided to a load coupled to the switching power converterplus inherent energy losses.